this is not accurate advice as there are multiple ticket types with different T&C and we do not know which ticket the OP has bought
there is a +1 hour window on a
standard ticket - the ticket is only cancelled if you are more than an hour late. Arrive 50 mins after departure time booked and you will still be offered a new slot if it is too late to get on the booked train.
also, if the ticket was bought before the new T&C came into effect in March this year, then the old t&C apply. This is causing a lot of confusion in some places as people post they were allowed a 2 hour window for free yesterday. Although, I guess the OP has a newly bought "last minute" ticket but we do not know if it is the standard, std plus, flexi or one of the very restricted tickets.
Anyone travelling on Le Shuttle should read
THIS that details the T&C for different ticket types